The invention pertains to a furnace and a stoker for a furnace which uses combustion air supplied by a blower, and fuel fed by a conveyor screw inserted into a hot air heat exchanger. The furnace is particularly adaptable for use in domestic and commercial heating, but is also adaptable in commercial and agricultural drying applications.
In prior art furnaces, solid fuels such as bio-mass pellets and coal are burned in a number of ways. Stoker assemblies are provided which convey fuel to the surface of a rotating annular grate disposed in an angular plane allowing accumulated ash to fall off the lower edge of the grate by gravity. Such assemblies result in the formation of large clinkers when burning coal and are also inefficient with regard to achieving complete combustion of the fuel.
In other types of the devices, a fire port is utilized having an enclosed lower retort portion which continues upward and has openings which constitute a tuyere. Clinker formation is enhanced when coal is burned inside such an enclosure. In such devices in which combustion air is fed into the sides or bottom of the retort, there is a risk that, during the idle cycle of the device, a cap of clinkers may form at the top of the pot diverting combustion air into the fuel conveyor mechanism, igniting fuel therein. If not discovered early, this process can continue until ignition progresses through the fuel conveyor to the fuel storage bin.
Another variety of stoker mechanisms utilizes a flat, stationary grate system and employs apparatus for fuel to enter one side of the grate, be transported across the grate upon which combustion takes place, and thence, as ash, to fall by gravity into a suitable container or removal conveyor. These devices are wasteful insofar as they do not achieve complete combustion of the fuel. Additionally, such devices, through the manner in which fresh fuel is co-mingled with burning fuel, create an inordinate amount of noxious emissions which generally cannot later be modified in the burning process and which thus enter the atmosphere as pollutants.
Some burning devices incorporate mechanism for secondary combustion of gases given off by heated fuels or partially burned fuels. These mechanisms are cumbersome, require periodic adjustment and a separate combustion air source also subject to adjustment, and are only partially successful in achieving secondary combustion of gases.
A further burning device incorporates a circular, horizontal stationary grate upon which a concentric ring with dependent teeth is caused to periodically rotate when engaged by a drive sprocket. The object of the concentric ring is to break up and discharge clinkers and ash formed in the combustion process. Because of the upward angle in which the drive sprocket tooth attacks the dependent teeth on the concentric ring, the ring is frequently lifted from the grate and becomes either jammed or dislodged causing malfunction of the entire apparatus.
A disadvantage shared by all stoker fed burning devices using combustion air supplied by a blower is the creation, in the burning process, of a quantity of finely divided particulate matter commonly called fly ash. In most such burning devices fly ash is caught up in the exhaust gases of the device and eventually is expelled into the atmosphere.
According to the present invention, there is provided a furnace having a primary heat exchange unit serving also as a combustion chamber, a secondary heat exchange unit connected by an upper crossover conduit to the primary heat exchange unit, and a tertiary heat exchange unit connected by a lower crossover conduit to the secondary heat exchange unit. A screw conveyor is provided to convey solid fuel such as coal to a burner assembly located in the combustion chamber. The burner assembly includes a combustion air housing carrying a circular, stationary grate having an annular valley for carrying fuel during combustion. The housing has a central opening which is connected to the fuel conveyor for introduction of fuel to the grate through the lower portion of the housing. The housing also has combustion air introduction conduits which introduce air under pressure at the lower portion of the grate. An agitator and discharge ring is situated on the grate and is rotated on the grate by a suitable drive sprocket mechanism connected to the screw conveyor. A horizontal burner plate is supported on the discharge ring by suitable legs and is disposed above the grate to promote more complete combustion of the fuel.
A fly ash removal assembly is located in the lower crossover conduit between the secondary and tertiary heat exchangers and includes an actuator for vibration of the walls of the heat exchanger to loosen fly ash and cause it to drop to the lower portion of the crossover conduit where it is carried by a fly ash conveyor screw to the primary heat exchange unit for disposal along with spent ashes.
The burner assembly provides for the inhibition of the formation of all but very small clinkers, an extremely high degree of reliability and operation, simple and relatively inexpensive construction, adaptability to a wide variety of sizes and uses, and substantially complete combustion of both solid fuel as well as combustible gases produced by burning solid fuels. Maximum heat recovery is achieved in the ommission of potentially dangerous pollutants into the atmosphere is minimized.
Additionally, fly ash is deflected away from the main current of the combustion gases emitted into the atmosphere as exhaust, and pollution from fine and particulate matter is substantially reduced.